The present invention relates to an objective lens for converging an electron beam which is suitable for use in an electron beam apparatus such as an electron beam length measuring machine, scanning electron microscope, or the like such that an electron beam is converged onto a specimen such as a semiconductor wafer or the like, the reflected electron or secondary electron from the specimen is detected, thereby measuring and evaluating a fine pattern on the specimen.
Hitherto, in the scanning type or transmitting type electron microscope, as disclosed in, e.g., JP-A-55-136446, there has been known an in-lens system in which a specimen is disposed between the upper and lower magnetic pole members of the objective lens, the objective lens is strongly excited, and an electron beam is converged onto the specimen. In this case, since the focal distance is reduced and the lens aberration can be reduced, a pattern on the surface of the specimen can be observed with the high resolution. However, the specimen which can be led into the objective lens is limited to a very small specimen having a diameter of about 3 to 6 mm. In addition, since it is necessary to move the specimen in the direction of the plane perpendicular to the electron beam and to be inclined to this plane, the specimen is disposed on the edge of a movable arm. However, this arm is easily vibrated by the external vibration and the position of the specimen easily fluctuates for the electron beam, so that it is difficult to obtain an image with a high resolution.
On the other hand, as an electron beam apparatus to measure and evaluate the specimen of a large diameter such as a semiconductor wafer (having a diameter of 8 inches), magnetic disc (having a diameter of 14 inches), or the like, the electron microscope shown in JP-A-55-136446 or the electron beam length measuring machine shown in "The International Society for Optical Engineering", 1985, August has been known. In this case, as shown in FIG. 1, a specimen 15 of a large diameter is disposed below a lower magnetic pole 12 of the objective lens 8 so as to be movable in the horizontal direction, thereby enabling the whole surface of the specimen to be observed. Further, the specimen 15 is arranged near the lower magnetic pole 12 as close as possible, thereby reducing the focal distance and decreasing the aberration of the lens. However, since the space between an upper magnetic pole 10 and the lower magnetic pole 12, the thickness of lower magnetic pole 12, and the space between the lower magnetic pole 12 and the specimen 15 exist, there is a limitation in reduction of the focal distance and the focal distance can be reduced to only a value of about 10 mm. Consequently, there is a limitation to obtain the high resolution.
In FIG. 1, reference numeral 2 denotes an electron gun; 4 is an electron beam; 6 a capacitor lens; 7 a deflecting coil; 10 the upper magnetic pole; 14 an exciting coil; and 16 a secondary electron detector.